1. Field of the Invention
The present invention relates to a fluid flow speed measuring apparatus. More specifically, the present invention is directed to fluid flow speed measuring apparatus for measuring a flow speed of a fluid flowing inside a large diameter pipe and containing entrained solids.
2. Description of the Prior Art
Devices for measuring a flow speed or flow rate of a fluid based on various principles have been proposed and put into practical use. Examples are an orifice-type flow rate measuring apparatus, a pitot-type flow speed measuring apparatus, a hot-wire current meter, an electromagnetic flowmeter, and the like. However, no apparatus has been developed which is suitable for measuring a flow speed of a flow rate of air and the like containing a large amount of entrained dust and the like and flowing inside a large diameter air-supply pipe or a combustion exhaust-gas pipe having an inner diameter of about 5 to 10 m and installed in an operating environment, e.g., an iron producing plant. In an orifice-type flow rate measuring apparatus, an orifice itself mounted on a large diameter pipe is not practical and dust inside the pipe adheres thereto so that a satisfactory measurement accuracy cannot be obtained. In a pitot-type flow speed measuring apparatus, a large diameter of a pipe is not a problem but clogging of a pitot pipe presents a problem. That is, a total pressure hole of the pitot pipe opened toward a flow so as to measure a total pressure of the flow is quickly clogged by dust contained in a fluid to be measured such as combustion exhaust gas. In a hot-wire current meter, which cools a thermistor or the like heated by Joule heat by a fluid to be measured and measures a flow speed on the basis of an extent of cooling, changes over time in measurement outputs are undesirably large because of adhesion of the dust in the fluid. In addition, this hot-wire current meter is liable to break because a metal filament or a thermistor as a sensor element is directly exposed into the fluid. Furthermore, in an electromagnetic flowmeter utilizing the Faraday's law of electromagnetic induction, as long as a fluid is electrically conductive, measurement can be performed even if the fluid has high viscosity or fine particles are mixed therein. However, since a fluid to be measured is limited to a conductive fluid, a flow rate of a nonconductive fluid such as air cannot be measured.